Sylvester Stallone was on David Letterman recently, not sporting a Panerai, which is his usual watch of choice, this time he is wearing a U Boat watch. The popularity of these timepieces is growing rapidly. He's wearing the U Boat Flightdeck 55 CAB O or the U Boat Flightdeck 50 CAB O, judging from his build it is probably the 55mm model.